A conventional hybrid vehicle uses an internal combustion engine and a rotary electric machine such as an electric motor/generator as drive power sources. The hybrid vehicle is adapted such that an in-vehicle battery may be charged by an electric power source, which is external to the vehicle. In this hybrid vehicle, the in-vehicle battery is charged externally by plugging an electric connector connected to the in-vehicle battery through an electric cable into an electric power outlet of a home (external power supply source). Such a vehicle is referred to as a plug-in hybrid vehicle.
The electric power supplied from the external power source to the plug-in hybrid vehicle is for example commercially available power such as commercial home electric power. The home electric power is lower in cost than engine fuel such as gasoline. Therefore, the cost to be borne by a user can be reduced as the electric power supplied externally of the vehicle is used more in driving the vehicle. Further, the number of times of refilling fuel at gas stations can also be reduced.
According to a plug-in hybrid vehicle disclosed in patent document (EP 1920986 corresponding to JP 2007-62638A), the output power of an internal combustion engine is not used to drive a motor/generator for generation of electric power until a state-of-charge (SOC) of an in-vehicle battery falls below a predetermined reference level. Under this condition, the vehicle is driven to travel in an electric vehicle (EV) travel mode by stopping the engine and driving the motor/generator as a motor with electric power stored in the in-vehicle battery. Thus, the plug-in hybrid vehicle uses electric power supplied externally, that is, low-cost energy, with priority. The vehicle is driven in a hybrid vehicle (HV) travel mode by driving the motor/generator as a generator to generate electric power by using the output power of the engine, when the SOC falls below the predetermined reference level. Under this condition, the vehicle travels while repeating charging and discharging of the battery so that the SOC may be maintained between predetermined upper limit and lower limit.
The hybrid vehicle according to the patent document is driven in the HV travel mode only after it travels more than a certain distance with the electric power supplied by the external power source and the SOC of the battery falls below the predetermined reference level. That is, it is only after the SOC of the battery falls below the predetermined reference level, that the motor/generator is driven by the output power of the engine to generate electric power.
If the motor/generator is driven to generate electric power by the output power of the engine, it is preferred that the amount of fuel consumed by the engine to generate the electric power is as small as possible. The amount of fuel required to generate the electric power however varies with vehicle travel conditions such as drive power and travel speed of the vehicle.
If the HV travel mode is limited to only a case, in which the SOC becomes less than the predetermined reference level, the total cost of energy (fuel and electricity) required to vehicle travel will not be minimized. For instance, electric power will not be generated with smaller amount of fuel, if a planned travel route of a vehicle includes many areas in the former part of the planned travel route, where the electric power can be generated with high efficiency (low fuel consumption).